Processing speed is important, in industrial laminating and other applications for cost considerations and energy conservation. In industrial laminating, the substrate, e.g., paper, is drawn through a dip tank containing a phenol-formaldehyde resole resin binder solution, then drawn continuously through a heated oven to remove most of the volatile material and advance the resin somewhat. The faster the rates the more efficient the binder and process, providing great utility.
Several ways to improve processing speed are known. One is to increase the molecular weight of the resin and the other is to use higher solids resin. Both methods suffer because of penetration problems into the substrate, leading to poorer final laminate appearance and greater water absorption which also gives poorer electrical properties. Generally, a low viscosity resin solution and/or low molecular weight resin is desired for substrate penetration and the molecular weight of the resin is raised during the heating step. Increasing the molecular weight in the impregnated substrate prepreg is needed so that when the prepreg layup is cured under heat and pressure, excessive resin flow out of the layup is not encountered.
It has now been discovered that a low viscosity binder composition comprising a resole resin and a metal salt curing accelerator will provide rapid impregnation of substrates with rapid advancement during drying so that during lamination and curing, excessive resin flow from the laminate is not experienced. It has been further discovered that the particular metal salt curing accelerators provide accelerated curing rates without adversely affecting electrical properties.